This invention relates in general to ribbon drive mechanisms, and in particular, to ribbon drive mechanisms for ink ribbons employed in a ribbon cartridge in printing applications.
Ribbon drive mechanisms are known in the art as disclosed in U.S. Pat. No. 4,011,933. This prior art cassette has a drive roller with a driver supported in fixed position in the cassette casing and a free floating pressure ring on a bearing stud with play between the pressure ring and the driver. The drive roller is positioned on the receiving or rolling-up side of the bearing stud and pulls the ink ribbon from a storage roll within the cassette through a printing location. Tension of the ribbon wedges the pressure ring between the bearing stud and the driver to create pressure of the pressure ring on the drive roller with the frictional connection between ribbon and drive roller increasing with increasing ribbon tension.
This prior art mechanism has been satisfactory; however, it suffers from the disadvantage that threading of the ink ribbon between the pressure ring and drive roller is very difficult when the drive roller has flanges to guide the ink ribbon which necessarily overlap the pressure ring. Another disadvantage is the sliding friction of the pressure ring on the bearing stud, which also increases with increasing ribbon tension. Because of this undesirable sliding friction, the pressure ring must have a low coefficient of friction, which greatly limits the material selection. It is also desirable that for interchangeable uses of the ink ribbon to provide a cassette which can be used at different ribbon advance speeds. This necessitates using drive rollers having different diameters. This is possible in the known cassette only by shifting the position of the bearing stud in relation to the axis of the drive roller. Therefore, such a solution requires another casing for each reduction or speed change.
Accordingly, it is desireable to provide a drive mechanism for an ink ribbon cassette which overcomes the shortcomings of the prior art device described above.